Abstract
AbstractIn the Cadomian orogenic belt a package of glacigenic sedimentary deposits have been recently described in the Armorican Massif (Normandy, France). The Granville Tillite Member, the middle part of the upper Granville Formation, is late Ediacaran in age. Maximum depositional ages of the pre- and syn-glacial sedimentary deposits obtained by LA-ICP-MS U–Pb detrital zircon dating indicate a maximum age of 561 ± 3 Ma. Combined with geochronological data on the previously described glacial deposits in Cadomia, West Africa, Arabia and Iran, the Granville Tillite Member appears to represent an Upper Ediacaran Glacial Period in northern peri-Gondwana, clearly younger than the c. 580 Ma old Gaskiers glaciation. Detailed mapping and analysis of the depositional regime of two sections near the city of Granville are indicative of two independent glaciomarine lower and upper tillite deposits separated by a distinct conglomeratic marker horizon, evidently a massive gravel beach horizon deposited during an interglacial stage. Age spectra of detrital zircon U–Pb ages constrain the palaeogeographical position of the upper Granville Formation to the periphery of the West African Craton. Post-Gaskiers aged glaciations in Cadomia and in West Africa should be grouped into an Upper Ediacaran Glacial Period dated at c. 565 Ma. This glacial period seems not to be related to the negative δ13C Shuram–Wonoka anomaly. Sedimentary deposits formed during the Upper Ediacaran Glacial Period show a scattered distribution along the marginal orogens of the Gondwana supercontinent independent of palaeolatitude and are coupled most likely to contemporaneous orogenic processes and uplift.
Highlights
Glaciations have had major impacts on the evolution of life on planet Earth (Hoffmann et al 2004, 2017)
Sedimentary features, aspects of basin development, the plate tectonic framework and U–Pb ages, all considered together, allow the reconstruction of Ediacaran glacial processes, palaeogeography and timing of orogenic processes in that part of the Cadomian orogen, which is preserved in the Granville Formation (North-Armorican Domain, Normandy, Armorican Massif)
Sedimentary and other features of the deposits further allow tracing a glacial period within sedimentary rocks related to the Cadomian orogen
Summary
Glaciations have had major impacts on the evolution of life on planet Earth (Hoffmann et al 2004, 2017). We present new sedimentological and geochronological data from the diamictite-bearing sedimentary succession of the upper Ediacaran Granville Formation in the Armorican Massif (Cadomian orogenic belt of peri-Gondwanan Europe, France; Fig. 1). Such sedimentary units are part of the upper Granville Formation and are characterized by uniquely glaciomarine features. Maximum depositional ages of detrital zircons in this formation allow us to constrain the succession to late Ediacaran and post-Gaskiers time. Eyles (1990), to the contrary, suggested the sedimentary deposits of the Granville Formation were debris flows and ‘gravel turbidites’ with no glacial influence
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